1. Field of the Invention
The present invention relates generally to particle fuel burning furnaces and, more particularly, is concerned with an improved fuel diversion structure defining a flow path from the upper chamber to the lower chamber which increases the dwell time of flow entrained particles in the upper chamber so as to promote combustion thereof before they reach the lower chamber.
2. Description of the Prior Art
In times of constantly increasing energy costs, the utilization of waste materials as fuel to produce energy is of increasing importance. Waste materials are amply available from various sources, for example, agricultural, forestry and industrial operations.
Many different furnaces (including incinerators and the like) appear in the prior art for burning conventional types of fuel, such as coal and wood, as well as waste or by-product types of particle fuel, such as sawdust, pulverized trash and wood chips. Representative of the prior art are the furnaces disclosed in Barnett (U.S. Pat. No. 2,058,945), Evans (U.S. Pat. No. 3,295,083), Midkiff (U.S. Pat. No. 3,822,657), Kolze et al (U.S. Pat. Nos. 3,865,053; 4,311,102; 4,377,115), Culpepper, Jr. (U.S. Pat. No. 3,932,137), Leggett et al (U.S. Pat. No. 3,951,082), Probsteder (U.S. Pat. No. 4,218,980), Hill (U.S. Pat. No. 4,309,965), Smith et al (U.S. Pat. No. 4,312,278), Payne et al (U.S. Pat. No. 4,378,208), Voss (U.S. Pat. No. 4,385,567), Ekenberg (U.S. Pat. No. 4,430,949) and Ingersoll et al (U.S. Pat. No. 4,479,481).
Another prior art furnace for burning waste product particle fuel is manufactured by Eshland Enterprises, Inc. of Greencastle, Pa. under the trademark "Wood Gun". Generally referred to as a wood gasification boiler, it has an insulated housing in which an upper, primary particle fuel retention and combustion chamber and a lower, secondary or afterburning combustion chamber are formed by refractory materials. A series of generally vertically extending passageways interconnect the bottom of the upper chamber with the top of the lower chamber. A quantity of waste particle fuel delivered into the upper chamber of the boiler through a fuel inlet in the top of the housing falls toward the bottom of the upper chamber and forms into a pile of fuel particles. The pile of particle fuel is ignited and burns from the bottom adjacent the location of the passageways. Periodically, the pile is replenished by delivery of additional particle fuel through the top fuel inlet of the housing. The portions of the refractory material containing the passageways are formed as separate removable bricks which can be replaced if they should deteriorate due to flame erosion over extended use without having to replace the whole of the refractory material.
Combustible gases generated as by-products from the burning of the particle fuel in the upper, primary chamber, along with air introduced into the upper portion of the primary chamber above the pile of fuel, are drawn downward through the passageways into the lower, secondary chamber by a draft inducing fan which creates a negative pressure drop in the lower chamber relative to the upper chamber. A suitable heat recovery unit is connected to the lower combustion chamber for capturing much of the heat produced by burning the combustible gases therein.
Notwithstanding the fact that the abovedescribed boiler has proven to be an efficient and economical way to convert waste products into usable heat energy, several problems have arisen which can adversely affect the long term operation of the furnace. First, some particles of fuel fall through the passageways into the lower combustion chamber during the normal course of operation with the result that air flow and combustion are impeded, thereby reducing the heat output of the furnace. Frequent cleaning is then required to remove the material deposited in the lower combustion chamber. Secondly, high temperature flames passing across the edges of the passageways cause spalding and erosion of the edge surfaces with the result that the passageways gradually become enlarged with continued use. When the wearing process has proceeded for some time, the passageways become so large that larger pieces of fuel will fall through the passageways, thus requiring premature replacement of the refractory bricks which incorporate the passageways.
To eliminate the occurrence of the abovementioned problems, a particle fuel diversion structure as illustrated and described in the above crossreferenced application was developed. When placed at the bottom of the upper chamber in spaced relationship above the passageways, the diversion structure creates a slot extending away from and upstream of the passageways which prevents most small size particles of fuel from falling through the passageways or from being drawn into the lower combustion chamber by the downdraft. Also, flame erosion now takes place on the edges of the diverter block of the diversion structure rather than in the passageways. The service life of the lower combustion chamber is greatly extended, and the relatively inexpensive fuel diverter block can be easily and conveniently replaced rather than the expensive refractory material. Also, the fuel diversion structure has the advantage of permitting the burning of finer particles, such as sawdust, shavings, and biomass pellets, than was possible heretofore.
While the fuel particle diversion structure has substantially reduced the earlier problems of refractory material flame erosion and fuel particle infiltration into the lower combustion chamber in most ordinary applications of the furnace, the burning of fuel particles of a very small size, such as pulverized wood waste fuel, has still proved to be impractical. The downdraft inducing fan which is employed to create a negative pressure in the primary combustion chamber also has the effect of aspirating the very small and light particles of fuel from the primary chamber and depositing them in the lower combustion chamber. Unless this effect is impeded in some manner, the lower chamber and ash recovery cyclone located downstream thereof rapidly fill with incompletely burned particles which necessitates very frequent cleaning. Consequently, a need still exists to make certain improvements which will facilitate combustion of very small fuel particles in the primary chamber of the furnace.